The present invention relates to an irradiation system and method, and more particularly to a system for irradiating material inside a sealed conduit that houses a conveying system.
Irradiation technology for medical and food sterilization has been scientifically understood for many years dating back to the 1940's. The increasing concern for food safety as well as safe, effective medical sterilization has resulted in growing interest and recently expanded government regulatory approval of irradiation technology for these applications. United States Government regulatory agencies have recently approved the use of irradiation processing of red meat in general and ground meat in particular. Ground meat such as ground beef is of particular concern for risk of food borne illness due to the fact that contaminants introduced during processing may be mixed throughout the product including the extreme product interior which receives the least amount of heat during cooking. Irradiation provides a very effective means of reducing the population of such harmful pathogens.
The available sources of ionizing radiation for irradiation processing consist primarily of gamma sources, high energy electrons and x-ray radiation. The most common gamma source for irradiation purposes is radioactive cobalt 60 which is simple and effective but expensive and hazardous to handle, transport, store and use. For these reasons, electron beam and x-ray generation are becoming the preferred technologies for material irradiation. An exemplary maximum electron beam energy for irradiation purposes is on the order of 10 million electron-volts (MeV) which results in effective irradiation without causing surrounding materials to become radioactive. The necessary electron beam power must be on the order of 5 to 10 kilowatts or more to effectively expose materials at rates sufficient for industrial processing.
Electron beam and x-ray irradiation systems both employ an electron accelerator to either emit high velocity electrons directly for irradiation or to cause high velocity electrons to collide with a metal conversion plate which results in the emission of x-rays. A number of electron acceleration techniques have been developed over the past several decades including electrostatic acceleration, pumped cylindrical accelerators and linear accelerators.
Electrostatic accelerators are characterized by the use of a direct current static voltage of typically 30 to 90 kilovolts which accelerates electrons due to charge attraction. Electrostatic accelerators are limited in maximum energy by the physical ability to generate and manage high static voltage at high power levels. Electrostatic accelerators using Cockroft-Walton voltage multipliers are capable of energy levels of up to 1 MeV at high power levels, but the 10 MeV energy level utilized by many systems for effective irradiation is not typically available.
Cylindrical electron beam accelerators have been in use for a number of years. These accelerators generally operate by injecting electrons into a cylindrical cavity, where they are accelerated across the cavity by radio frequency energy pumped into the cylinder and redirected across the cavity by magnets to be further accelerated. Once the electrons reach a desired energy level, they are directed out of the cylinder toward a target.
RF linear accelerators have also generally been in use for many years and employ a series of cascaded microwave radio frequency tuned cavities. An electron source with direct current electrostatic acceleration injects electrons into the first of the cascaded tuned cavities. A very high energy radio frequency signal driven into the tuned cavities causes the electrons to be pulled into each tuned cavity by electromagnetic field attraction and boosted in velocity toward the exit of each tuned cavity. A series of such cascaded tuned cavities results in successive acceleration of electrons to velocities up to the 10 MeV level. The accelerated electrons are passed through a set of electromagnets that shape and direct the beam of electrons toward the target to be irradiated.
A typical industrial irradiation system employs an electron beam accelerator of one of the types described, a subsystem to shape and direct the electron beam toward the target and a conveyor system to move the material to be irradiated through the beam. The actual beam size and shape may vary, but a typical beam form is an elliptical shape having a height of approximately 30 millimeters (mm) and a width of approximately 45 mm. The beam is magnetically deflected vertically by application of an appropriate current in the scan deflection electromagnets to cause the beam to traverse a selected vertical region. As material to be irradiated is moved by conveyor through the beam, the entire volume of product is exposed to the beam. The power of the beam, the rate at which the beam is scanned and the rate that the conveyor moves the product through the beam determines the irradiation dosage. Electron beam irradiation at the 10 MeV energy level is typically effective for processing of food materials up to about 3.5 inches in thickness with two-sided exposure. Conversion of the electron beam to x-ray irradiation is relatively inefficient but is effective for materials up to 18 inches or more with two-sided exposure.
In addition to food materials, recent attacks on the United States Postal Service (USPS) have occurred in which highly dangerous Bacillus anthracis spores have been placed in envelopes and mailed, suggesting a need for irradiation of mail and related paper materials. The levels of radiation exposure that may be used to sanitize mail are significantly higher than those allowable for food. This is due to the fact that mail materials typically consist of paper and ink and are not intended to be consumed by individuals. The doses of radiation may therefore be set to relatively high levels that are effective in eliminating both spore forming bacteria such as Bacillus anthracis as well as viruses. Each of these types of pathogens are relatively resistant to ionizing radiation; spore forming bacteria because of the compactness and durability of the spores; and viruses due to the relatively small size of viral DNA molecules. The target dose established by the USPS is 56 kGy, which is 8 times higher than the maximum allowable dose for irradiation of frozen meat and 37 times higher than the typical dose applied to fresh ground beef for the elimination of E. coli. The considerations that must be accounted for in irradiating paper materials are somewhat different from those relating to irradiation of food materials, in large part due to the higher dose requirements and smaller cross sectional thickness of typical mail, but several concepts and configurations are applicable to both food and paper irradiation.
There are a number of prior art irradiation systems that utilize accelerators and conveying systems of some kind in a highly effective manner to irradiate articles and/or bulk material. Two such systems are described in U.S. application Ser. No. 09/685,799 filed Oct. 10, 2000 for “Irradiation System and Method” by S. Lyons, S. Koenck, B. Dalziel and J. Kewley and in U.S. application Ser. No. 09/795,058 filed Feb. 26, 2001 for “Bulk Material Irradiation System and Method” by S. Lyons, S. Koenck, B. Dalziel, D. White and J. Kewley, both of which have been incorporated by reference herein. Although irradiation systems such as these employ a number of useful features, additional features may be desirable for different types of irradiation applications. The present invention provides a number of features not previously known or described in the art, which are described in detail below.